slate in a 100 gallon

A bunch. Just make sure that it's not on a point so the weight is distributed.

plenty, espiecially if its for african cichlids. make sure the bottom of the tank has plenty of support so it can hold it all.

If you are putting a lot of slate into a tank a good idea is to add egg crating to the obttom of your tank prior to placing the slate. This will help protect your glass from any 'accidents' while putting in the slate. You should also put in the slate prior to putting in your substrate as this will help give it that bit more realistic look as well as provide a bit more stability as the slate is all weighted down on the egg crate and not moveable substrate. This is especially adviseable if you will have any earth moving fish in the tank.